It should depend on what their diagnosis was.
Obviously someone with a history of homocidal tendancies should not be allowed weapons...or close contact with other people.But who gets to decide what constitutes 'mental illness'?The 'experts' who rely largely on their own research? The courts that have a long history of infringing on,or outright stripping the rights of people who come through their courtroom? This seems like a left wing trap to me. However you vote,they will use it to take away someone's right to be armed,or to vilify that right entirely.
I won't vote on this one.